As one can ascertain, the prior art is replete with many devices which operate to monitor a fluid level in a tank or other storage vessel. Many devices operate by means of an electromechanical switch which is activated by a float. The float rides or floats on the surface of the fluid. When the fluid reaches a given level, a switch coupled to the float is operated. These types of devices are undesirably subject to sticking in either the on or off position though.
There are other systems which utilize electrical or electronic devices to detect the level of fluid in a vessel or otherwise. For example, fluid level controls for containers such as boilers are known as shown by U.S. Pat. No. 3,834,357 issued Sep. 10, 1974 to L. Kaczmarek et al. Conductance probe controls detect a level of a conductive media, such as a fluid, by passing an electrical current through a probe, into the conductive media and then back to a controller. The probe is at least partially immersed in the fluid when the fluid is at certain levels and not in contact with the fluid when the fluid is at lower levels. The controller measures the received current level to determine whether the probe is in contact with the media, i.e. whether a fluid is at or above a certain level. Such devices conventionally utilize AC currents, as applying a constant DC current to a probe exposed to a conductive fluid can undesirably promote probe deterioration. However, a drawback of using AC is that it requires the inclusion of support circuitry when also using a conventional microcontroller. This increases the cost and complexity of such a device.
To promote safety, probe excitation should also be limited to safe levels. Level control circuits in which the electrical path through an electrode or probe is at a low AC level such as 24 VAC is shown by U.S. Pat. No. 4,019,067 issued Apr. 19, 1977 to R. Gladstone.
It is an object of the present invention to provide a cost efficient fluid level sensor which overcomes these limitations of the prior art and yet allows for a sufficiently low probe excitation potential so as not to create an unnecessary safety risk.